The present invention relates to an electrode for use in an electrochemical capacitor. More particularly, the electrode of the present invention is ideal for use in an electrochemical capacitor of high energy storage capacity, wherein the capacitor has a double electric layer. Such capacitors are often referred to as “ultracapacitors” or “supercapacitors,” however, they will simply be referred to herein as “capacitors.”
There is an increasing focus on the use of capacitors as a means for storing electrical energy. These capacitors can efficiently store and redistribute a large amount of electrical energy. For purposes of illustration, and not limitation, such capacitors may be used: as a main power supply at a particular location; as a back-up power supply at a particular location; for power quality assurance (i.e., to compensate for short-term power “surges”, “spikes”, and “skips” common to a utility-supplied source of electrical power); to provide load-leveling by storing an amount of electrical energy provided during off-peak hours and thereafter re-distributing said electrical energy during periods of peak demand; and as a primary or secondary power source for a variety of vehicles.
A double electric layer (DEL) capacitor typically comprises a pair of electrodes residing in a spaced apart relationship, between which is an electrolyte. The electrolyte can be either aqueous or non-aqueous in nature, depending on the composition of the electrodes. A separator typically also resides in the space between the electrodes. One or both of the electrodes may store electrical energy through a double layer electrochemical mechanism. In a double electric layer storage process, a layer of electrons forms at the electrode side of the electrode/electrolyte interface. A layer of positive ions also forms on the electrolyte side of the electrode/electrolyte interface. The voltage across the electrode/electrolyte interface increases with charge accumulation, and is eventually released during discharge of the capacitor.
One or both of the electrodes of a DEL capacitor may generally be polarizable electrodes. The polarizable electrode may comprise, for example, an active material and a current collector to which the active material is affixed. The most commonly employed active material is likely one of a plurality of activated carbon materials. Activated carbon materials are inexpensive and have a high specific surface area per unit mass. Electrodes are typically formed from activated carbon materials in the form of an activated carbon powder and a binder, or from woven or non-woven activated carbon fiber materials. However, preparation of DEL electrodes from an activated carbon powder is often preferable due to its lower cost.
As stated above, in a typical capacitor, one or both of the electrodes may be polarizable. However, it has been found that constructing a DEL capacitor with one polarizable electrode and one non-polarizable electrode provides the DEL capacitor with a specific energy capacity that is greater than that of a capacitor with two polarizable electrodes. In such a DEL capacitor, charge storage at the non-polarizable electrode occurs as a result of oxidation and reduction reactions at the interface of the non-polarizable electrode and the electrolyte. Such an electrode is commonly said to exhibit Faradaic pseudocapacitive behavior.
Each of the electrodes of such a DEL capacitor is typically affixed by some means to a current collector. Current collectors are commonly constructed of a material that exhibits electrical conductivity—typically a metal. As at least a portion of the current collector, along with the electrode material, must reside in the electrolyte, it must be ensured that the current collector material will not react adversely thereto. For example, the electrolyte of a DEL capacitor may consist of an aqueous sulfuric acid. In such a case, certain precautions such as, for example, coating or otherwise protecting the portion of the current collector exposed to the electrolyte must generally be undertaken, as the sulfuric acid electrolyte may corrode or erode the current collector material.
While various embodiments of DEL capacitors are currently known, each typically has one or more inherent disadvantages. For example, the activated carbon powders used to form the electrodes of common DEL capacitors are often derived from the processing of coal raw material. Such an activated carbon powder will generally exhibit a high ash percentage (e.g., 15 weight percent or more). Additionally, these activated carbon powders also typically contain an unacceptably high quantity of admixtures of transition metals. This high ash percentage and large quantity of admixtures of transition metals present in the activated carbon powder will, of course, eventually become a part of any electrode formed therefrom. The presence of these impurities in an electrode limits the voltage to which a DEL capacitor employing the electrode can be charged. For example, the presence of admixtures of transition metals can reduce the decomposition voltage of an acid electrolyte and, thereby, decrease the operating voltage of a capacitor.
There are also other disadvantages to known DEL capacitor designs. For example, many of the activated carbon materials employed to form the electrodes of such capacitors require the addition of a large quantity of binder material. The use of more binder material results in a corresponding reduction in the amount of activated carbon material present in the resultant electrode. A reduction in the amount of activated carbon present in the electrode, subsequently diminishes the capacitance and electrical energy storage capabilities of a capacitor to which the electrode is installed. Additionally, steel and similar metals are often used to form the current collectors of a DEL capacitor. Unfortunately, steel and many other metals are not resistant to an acid electrolyte. For example, in the presence of a sulfuric acid electrolyte, a steel current collector will degrade, such as by corrosion. Corrosion of the current collectors can have a negative effect on the cycling capacity and service life of a capacitor. Consequently, to reduce or avoid degradation of such current collectors, known DEL capacitor designs have employed a protective coating that is resistant to the electrolyte used in the capacitor. The protective coating, depending on its composition, can be applied to the current collector by a variety of methods. As one example, a steel current collector may utilize a protective layer of graphite foil. While certain of these coating materials may offer acceptable resistant to the electrolyte in which they reside, there has been a great deal of difficulty in obtaining adequate adhesion between the protective coatings and the subjacent electrode materials. As a result, the electrolyte will often eventually intrude between the protective coating and the current collector. It should be realized that any degradation or erosion of such a metal current collector can adversely effect performance of a DEL capacitor. For example, when a sulfuric acid electrolyte is used, even substantially insignificant quantities of iron present therein can harshly decrease the decomposition voltage of the electrolyte and result in a significant reduction in the operating voltage of the capacitor. Hence, the degradation of the current collector should be avoided.
As can be understood from the foregoing discussion, there are several disadvantages associated with known DEL capacitor designs. The electrode of the present invention utilizes an improved design that substantially reduces or eliminates many of the problems associated with known DEL capacitors. The design of the electrode of the present invention can be used to produce a DEL capacitor having an increased specific energy, better reliability, greater cycling capacity, and an increased service life.